What's a Tropical Depression? Is it Different From a Tropical Storm?

Abhijit Naik Jun 16, 2019
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Our knowledge about the formation of tropical depression is restricted to weather reports, which is sad considering that the entire formation process is quite interesting in itself.
A tropical cyclone is a disturbed state of the atmosphere, most often characterized by a low pressure center, which eventually results in strong winds and heavy rains.
Based on their intensity, tropical cyclones are categorized into three groups: tropical depressions, tropical storms, and cyclones. Depending on their location, cyclones are also known as typhoons and hurricanes.

Tropical Depression

A stage of a tropical cyclone, it is an organized system of clouds and thunderstorms with a surface circulation of less than 39 mph. Though the depression doesn't have an 'eye', or the spiral shape which characterizes powerful storms, it is indeed a low pressure area and hence, is referred to as a depression.
The wind speed is measured at a height of 10 m above the water surface and averaged over a minute for data calculation. This depression mostly forms in the tropical regions of the planet, in maritime tropical air masses, and hence, it is referred to as a tropical depression. While tropical storms are named alphabetically, these depressions are numbered.

How does a Depression Form?

Cyclones most often form in the tropical areas, 5° and 20° North and South of the Equator, owing to the Coriolis effect.
Initially, a low pressure zone is formed in this area, which eventually triggers a tropical wave of low pressure. This disturbance in the warm ocean water, along with light wind at the upper level, provides ideal weather condition for the formation of storms.
With time, the intensity of the tropical wave may either increase or decrease. If the intensity increases, it becomes an organized area, characterized by heavy showers and thunderstorms. The organized area, marked by cyclonic winds blowing clockwise in the Northern Hemisphere and anti-clockwise in Southern Hemisphere, is referred to as tropical depression.
Owing to the lack of rotary motion of wind, equatorial areas 5° North and South of the Equator are not conducive for the formation of storms.

Tropical Strom

A tropical storm is a stage of tropical cyclone characterized by an organized system of strong thunderstorms with surface circulation between 39 mph and 73 mph.
Though the 'eye' is not present, the spiral cyclonic shape does start developing in this stage. When the system reaches this intensity, it is assigned an alphabetical name by the authorities.
The storm system, which starts as a depression, eventually takes the form of a severe cyclone which can unleash havoc on the planet. One of the most devastating cyclones experienced in recent times, Hurricane Katrina is the best example of how dangerous cyclones can become.
Owing to the rapid climate change that the planet has witnessed over the last few decades, tropical depressions and cyclones have only become more frequent. In a bid to gain supremacy over nature, we have created environment which is suitable for these natural disasters. Sadly though, we are yet to realize our mistake.